1. Field of the Invention
The present invention relates to an interferometer that is applied to test a performance of a beam itself emitted from various optical equipment or optical components or a performance of various optical equipment or optical components themselves, for measuring a wavefront of a beam.
2. Description of the Prior Arts
Conventionally, various types of interferometers have been well-known for measuring a wavefront of a beam, one of which is disclosed in a patent document (Japanese Unexamined Patent Application No. SHO57-64139). An interferometer disclosed in this patent document is provided with a light source 10, collimating lens 11, test lens 12 that is a subject to be tested and semi-transparent substrate 13 as shown in FIG. 13, wherein divergent light emitted from the light source 10 is converted into parallel beam by the collimating lens 11 and the converted parallel beam is focused on the semi-transparent substrate 13 by the test lens 12.
Arranged between the test lens 12 and the semi-transparent substrate 13 are a diffraction grating 14 for forming plural focal spots on the semi-transparent substrate 13 by plural diffracted lights of zeroth-order, plus and minus first-order or the like and a field stop 15 that blocks the diffracted light of more than zeroth-order and plus first-order, whereby the focal spots by the zeroth-order diffracted light and first-order diffracted light are formed on the semi-transparent substrate 13. A pinhole 13a is formed at the focal position of the first-order diffracted light on the semi-transparent substrate 13. A reference wavefront generated by the first-order diffracted light passing through the pinhole 13a and a wavefront generated by the zeroth-order diffracted light passing through the semi-transparent substrate 13 are overlapped with each other on a field lens 16, thereby forming interference fringes on an imaging device 17.
However, the pinhole 13a for producing the reference wavefront is formed on the semi-transparent substrate 13 in the above-mentioned conventional device, so that a part of the first-order diffracted light produces the reference wavefront on the pinhole 13a but a part thereof passes therethrough with a wavefront including an aberration. As a result, a contrast of the interference fringes observed on the imaging device 17 is remarkably deteriorated, thereby deteriorating an accuracy of the wavefront measurement by this interferometer. In case where a coherency of the light source is low such as a semiconductor laser, in particular, a sufficient contrast cannot be obtained in the interference fringes.